This invention relates to a system and a method for compression braking within a vehicle having a variable compression ratio engine and more particularly, a compression braking system and method which utilizes the advantages and flexibility of a variable compression ratio engine to selectively augment and/or vary the produced braking torque.
Compression braking systems are typically employed within relatively large or commercial type vehicles and are effective to augment the torque provided by conventional friction braking assemblies and/or to regulate the speed of such vehicles. Compression braking is typically activated after the fuel supply has been xe2x80x9ccut offxe2x80x9d from the engine, such as when the vehicle is descending a steep grade. Compression braking is performed by providing resistance to airflow within the engine by changing valve timing and/or by other techniques. For example and without limitation, compression braking can be performed by opening a cylinder valve (e.g., an exhaust valve or a supplemental valve) at the end of a stroke, thereby releasing the potential energy stored in the compressed air into the atmosphere. In this manner, energy is removed from the driveshaft, thereby slowing the vehicle. Prior compression braking systems typically include several hydraulically actuated valves which communicate with various cylinders within the vehicle""s engine and which are selectively opened to release compressed air from the cylinders.
Efforts have been made to improve the efficiency and fuel economy of a vehicle engine by selectively varying the compression ratio within the various cylinders of the engine. For example and without limitation, a variable compression engine may selectively alter the compression ratio within its various cylinders by opening and closing auxiliary chambers within the cylinder heads, or by varying the length of the pistons and/or piston rods. These types of engines provide greater flexibility, improved fuel economy and reduced knock. However, vehicles having these types of engines are typically braked using conventional friction brakes and/or conventional compression braking systems and strategies. These conventional compression braking systems and methods do not utilize the flexibility provided by variable compression ratio engines, and therefore, are not well suited for variable compression ratio engines.
There is therefore a need for a new and improved system and method for compression braking which is adapted for use with a variable compression ratio engine and which utilizes the flexibility of a variable compression ratio engine to controllably vary or augment braking torque and to regulate the speed of a vehicle.
A first non-limiting advantage of the invention is that it provides a system and method for compression braking which overcomes at least some of the previously delineated drawbacks of prior systems, assemblies and methodologies.
A second non-limiting advantage of the invention is that it provides a system and method for compression braking which is adapted for use in combination with a variable compression ratio engine and which utilizes the compression braking function with the variable compression ratio function in a synergistic manner to achieve a continuously varying compression braking torque.
A third non-limiting advantage of the invention is that it provides a system and method for compression braking which utilizes compression ratio changes to augment compression braking capability.
According to a first aspect of the present invention, a system for braking a vehicle is provided. The system includes a variable compression ratio engine which is selectively operable in a high compression ratio mode and in a low compression ratio mode; a compression braking system which selectively provides a compression braking torque; and a controller which is communicatively coupled to the variable compression ratio engine and to the compression braking system, the controller being effective to activate the compression braking system and to selectively cause the variable compression ratio engine to switch between the high compression ratio mode and the low compression ratio mode while the compression braking system is activated, thereby selectively varying the compression braking torque.
According to a second aspect of the present invention, a method is provided for braking a vehicle of the type having a compression braking system and a variable compression ratio engine. The method includes the steps of: activating the compression braking system, effective to produce a braking torque; and selectively altering the compression ratio within the variable compression ratio engine, thereby altering the produced braking torque.
These and other features, aspects, and advantages of the invention will become apparent by reading the following specification and by reference to the following drawings.